This invention relates to a switching device for an electric circuit, and more specifically to a switching device using an insulated-gate field-effect transistor (IGFET) for switching, together with means for protecting the IGFET from a reverse current flow.
The IGFET of typical conventional make has a drain electrode connected to a drain region of a semiconductor substrate, a source electrode connected to both source and body regions of the substrate, and a gate electrode overlying a gate insulator on the surface of the body region exposed between the drain and source regions. The source electrode makes ohmic contact with not only the source region but the body region as well. As a consequence, the IGFET has a channel connecting the drain and source electrodes through a parasitic diode (sometimes referred to as a body diode or built-in diode) based upon the p-n junction between the drain and body regions, in addition to that through the body region. Assuming that the IGFET has a channel of n-type (n-channel), the parasitic diode is reverse biased, blocking current flow therethrough, when the drain electrode is higher in potential than the source electrode. However, the drain electrode may grow less in potential than the source electrode because of some operation of the electric circuit incorporating the IGFET or of a wrong connection between the electric circuit and the power supply such as a battery. The parasitic diode will be forward biased in that case, permitting a current flow therethrough. The drain-source current flow is uncontrollable by the control voltage applied between gate and source as long as a current flows through the parasitic diode. What is worse, any large current flow through the parasitic diode can lead to the destruction of the IGFET itself or of the electric circuit.
One conventional approach to how to avoid the current flow through the parasitic diode of the IGFET was a serial connection of the IGFET and an external diode (reverse-blocking diode), the latter being polarized opposite to the parasitic diode. This external diode may be formed either on the same substrate as the IGFET or on a separate one. The substrate became inconveniently bulky, and the resulting composite device expensive, in the former case. In the latter case, too, the combination of the IGFET and the discrete external diode became bulky and expensive. The serial connection of the external diode with the IGFET was itself disadvantageous because of a large power loss incurred by the flow of current of the same magnitude through the diode as through the IGFET. Furthermore, when the drain was less in potential than the source, that is, when a reverse voltage was being impressed, the current flow through the IGFET was uncontrollable by the gate voltage.
With a view to defeating the problems arising from use of the external diode in combination with the MOSFET of the noted prior art construction, Japanese Unexamined Patent Publication No. 7-15009 proposes an advanced planar MOSFET where the source electrode is in Schottky contact with the body region. The present applicant also suggested in Japanese Patent Application 2006-326811 a trenched IGFET in which the source electrode is in Schottky contact with the body region. The Schottky contact of the source electrode with the body region results in the creation of a Schottky diode that functions to block reverse current flow.
The present invention proposes a novel switching device incorporating IGFETs, each with a built-in Schottky diode, which are per se of the conventional design noted above. The equivalent circuit of each such IGFET can be drawn as at 14 and 15 in FIG. 2 of the drawings attached hereto. Although this figure is an illustration of one of the preferred forms of the switching device according to the invention, the prior art IGFET with the built-in Schottky diode will be explained hereinbelow with reference to FIG. 2 and as exemplified by the main switching IGFET 14 depicted therein.
The exemplified IGFET 14 comprises a FET Q1, two p-n junction diodes Da and Db, and a Schottky barrier diode Dc. The first p-n junction diode Da is a parasitic (body) diode based upon the p-n junction between n-type drain and p-type body. The second p-n junction diode Db is a parasitic (built-in) diode based upon the p-n junction between p-type body region and n-type source region. The Schottky barrier diode Dc is based upon the schottky junction between source electrode S1 and p-type body region.
Polarized to be reverse biased when the drain electrode D1 is higher in potential than the source electrode S1, the first p-n junction diode Da is connected in inverse parallel with the FET switch Q1. The second p-n junction diode Db on the other hand is opposite in polarity to the first p-n junction diode Da and is connected in series therewith.
In a more conventional IGFET not having the Schottky barrier diode Dc, this part of the device is short-circuited, so the second p-n junction diode Db has no function at all and therefore does not appear in the equivalent circuit. Opposite in polarity to the first p-n junction diode Da, the Schottky barrier diode Dc is connected in series with the first p-n junction diode Da and in parallel with the second p-n junction diode Db.
Let it be supposed that both second p-n junction diode Db and Schottky barrier diode Dc have a sufficient antivoltage strength. During application of a reverse voltage to this IGFET 14, when the source electrode S1 is higher in potential than the drain electrode D1, the reverse current will be blocked by the second p-n junction diode Db and Schottky barrier diode Dc. However, when the source electrode S1 is positive, and the drain electrode D1 negative, in potential, then the gate electrode G1 will have a positive potential owing to the parasitic capacitance between source electrode S1 and gate electrode G1.
As the gate electrode G1 thus becomes higher in potential than the body region, the depletion layer due to the p-n junction between the body and source regions will become locally so thin adjacent the gate electrode G1 (where the p-n junction is exposed) that the second p-n junction diode Db will suffer a loss in antivoltage strength. If the voltage between source electrode S1 and drain electrode D1 is very high, the second p-n junction diode Db will become so poor in antivoltage strength as to become virtually dysfunctional, permitting a reverse current flow therethrough. There will then be no merit at all accruing from the presence of the Schottky barrier diode Dc.